The word "it" refers to "righteousness" in 10:6. Here Paul describes "righteousness" as a person that can speak. Paul uses a question to emphasize the answer he is about to give. AT: "But this is what Moses says" (See: [[rc://en/ta/man/translate/figs-personification]] and [[rc://en/ta/man/translate/figs-rquestion]])
The word "mouth" refers to what a person speaks. This can be translated as a new sentence. AT: "it is in what you say" (See: [[rc://en/ta/man/translate/figs-simile]])